The critical interaction of glycoprotein (GP) Ib beta with GPIX - A genetic cause of Bernard-Soulier syndrome

Bernard-Soulier syndrome is an uncommon bleeding disorder caused by a quant itative or qualitative defect in the platelet glycoprotein (GP)Ib/IX comple x. The complex is composed of four subunits, GPIb alpha, GPIb beta, GPIX, a nd GPV. Here we describe the molecular basis of a novel Bernard-Soulier syn drome variant in a patient in whom GPIb alpha and GPIX were undetectable on the platelet surface. DNA sequence analysis showed normal sequence for GPI b alpha, GPIX, and GPV. The GPIb beta gene has been mapped to the 22q11.2 r egion of chromosome 22 which was deleted from one chromosome of this patien t. There was a single nucleotide deletion within the codon for Ala 80 in GP Ib beta within the other allele. This mutation causes a translational frame shift that encodes for 86 altered amino acids and predicts a premature sto p 15 amino acids short of the length of the wild-type protein. Transient co expression of the mutant GPIb beta in 293T cells with wild-type GPIb alpha and GPIX resulted in the surface expression of GPIb alpha, but the absence of GPIX. Moreover, when a plasmid encoding the wild-type GPIb beta was tran siently transfected into Chinese hamster ovary cells stably expressing GP a lpha, which retain the capacity to reexpress GPIX, there was a significant increase in the surface expression of GPIX. In contrast, when the mutant GP Ib beta was transiently transfected into these cells, GPIX was not reexpres sed on the plasma surface. Thus, a deletion of one copy of GPIb beta and a single nucleotide deletion In the codon for Ala 80 within the remaining GPI b beta allele causes the Bernard-Soulier phenotype through an interaction o f GPIb beta with GPIX resulting in the absence of GPIb alpha on the plasma membrane. The interaction of GPIb beta with GPIX is essential for the funct ional expression of GPI alpha. (C) 1999 by The American Society of Hematolo gy.